Target for x-radiation and the like



May l2, 1959 H. BERGER ErAL 2,886,726

- TARGET FoR x-RADIATIQN AND THE LIKE Filed Nov. 21, 1957 fain fAmss511m. (on CoPPBIZ I/'b Canam: on. Nuenen.)

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United States Patent O 2,886,726 TARGET FOR X-RADIATION AND THE LIKEApplication November 21, 1957, Serial No. 697,823 9 Claims. (CL 313-65)This invention relates to an improved target for X-radiation and thelike rendered photoconductive and capable of use in apparatus to producesharp X-ray images even though the objects from which the images areformed are moving at a relatively high rate. The invention is furtherconcerned with a method for produci such atarget.

Innireviously known devices of this type, lead oxide photoconductivematerial Ihas been applied to a substrate material such as aluminum,aluminum being capable of good X-ray transmission. (75 Electricallngineering 158-61, February 1956.) However, the oxide of aluminum hasvery high electrical resistance and the processing techniques which wedesire to use have been such as to produce in a non-uniform formation ofaluminum oxide between the substrate and the lead oxide coating,resulting in undesirable variation of photoconductivity characteristicsfrom point to point.

The present invention seeks to improve the target by use of ka substratewhich has at least its surface composed of stainless steel or one of anumber of other metal whose oxides have electrical resistance of th'eorder of 10 ohm-cm. or less, a good degree of electrical conductivitybetween the substrate surface and the lead oxide being assured, anduniformity enhanced.

The substrate desirably comprises a heavy metal relatively opaque toX-radiation, whereby to filter out soft scattered radiation and producesharper images than are otherwise obtainable. Moreover, the heavy metalintensites the photoconductive signal by means of photoemission from thesubstrate material, particularly at high X-ray energy in excess of 100kvp. Stainless steel is ideal, but it is necessary to select an alloyhaving a co-ecient of expansion sufficiently close to that of thecoating to preclude peeling. AISI type 430 is the most suitable alloypresently known. Since a form-sustaining sheet of stainless steel isquite opaque to X-ray, the preferred embodiments of the invention aremade by electroplating or otherwise coating with a thin film ofstainless steel a base plate of aluminum or other light metal which is agood electrical conductor and not unduly X-ray opaque. We have also usedcoatings essentially comprising copper, nickel or chromium or alloysthereof. However, stainless steel is preferred, particularly in medicalapplications.

Upon the substrate, lead oxide vapor is condensed and subsequently bakedto convert it to a yellow crystalline form of lead oxide. The bakingwill oxidize the heavy metal face of the substrate at the interface andit is important that the oxide of the substrate metal should shouldmatch that of the lead oxide sutliciently so that there will be nopeeling of the lead oxide layer. The substrates herein disclosed haveall of these properties and, additionally, are easily worked, relativelynon- ICC magnetic, and have good X-ray transmission. Additionally, thepreferred stainless steel substrate facing in particular is sufficientlyheavy to act as an electron intensifier. The target plate area has beenfound to be notably free of the spots or blemishes found in prior targetplates of this type. i

In the drawings:

Fig. 1 is a diagrammatic view showing a greatly enlarged fragmentarycross section of a target embodying the invention.

Fig. 2 is a diagrammatic view showing in greatly enlarged fragmentarycross section a preferred embodiment of a target incorporating theinvention.

Fig. 3 is a ow diagram of steps of the preferred method for producing atarget as shown in Fig. 2.

Fig. 4 is a diagrammatic view showing in cross section apparatus used tocoat the substrate with lead oxide.

In Fig. 1*, the substrate 5 comprises a sheet of stainless steel orchromium or nickel of just sucient thickness to be form sustaining. Uponthis is a layer 6 of an oxide of the metal of sheet 5 and aphotoconductive deposit 7 of yellow crystalline lead oxide.

In the embodiment of Fig. 2,' the substrate supporting sheet 8 isaluminum or other lightweight electrically conductive metal upon whichis bonded a thin deposit 50 of stainless steel preferably produced byplating or spraying it upon the substrate ply, as it is usually too thinto be self supporting. Heavier plies may be laminated to the sheet 8 byrolling. Upon the facing 50 is an oxide coating on which the lead oxidelayer or layers 9 and 10 are applied.

As already indicated, copper, nickel or chromium may be used in lieu ofstainless steel to provide a substrate surface in lieu of the stainlesssteel S of Fig. l or 8 of Fig. 2. Whatever the metallic sheet or filmused in the substrate to provide an interface with the lead oxide, itshould be of such a nature that the oxide formed at the surface andcontacted by the lead oxide should have electrical resistance verymaterially less than that of the lead oxide (which is l01l ohm-cm). Ithas been found satisfactory to use as the interface portion of thesubstrate a metal whose oxide has a resistance of the order of 1()6ohm-cm. or less. This insures proper electrical conductivity between thesubstrate and the deposit of lead oxide and gives results greatlysuperior to those achieved when the lead oxide is in contact withaluminum oxide or the like, the aluminum oxide having resistance greaterthan that of the lead oxide.

While the generic concept permits the use of a substrate sheet orsurface lm of copper, nickel or chromium, stainless steel, AISI type430, is the preferred material used at l5 in Fig. 1 and 8 in Fig. 2 toprovide the substrate surface. AISI type 430 is a preferred exampleprincipally because its co-etlcient of expansion so nearly-matches thatof lead oxide that no separation occurs during heating or coating.

As applied, the lead oxide contains a large percentage of the red formof lead oxide. It has been found that the yellow crystalline leadoxidegives faster response to X-radiation than the red form of lead oxide.The preferred method of applying the photo-conductive layer and itsconversion to the desired yellowcrystalline form of lead oxide is asfollows, reference being made to Fig. 3.

The substrate plate generically designated by reference character 12 maycomprise the sheet 5 of Fig. l or the supporting sheet 8 and surface lm50 of Fig. 2. The plate 12 is placed in a vacuum chamber 13 and over itis placed a heater 14 of the electrical resistance type having currentsupply leads 15 and 16. Gas is thereupon withdrawn from the chamber 13through the pipe 17 of vacuum is not critical, it has been foundappropriate to draw a vacuum of 10-4 mm. Hg to 10-5 mm. Hg. Dry oxygenis then introduced through the pipe 18 in such manner as to maintain apressure of 3 to 5 microns within the chamber. After the oxygen pressurehas remained stable for 5 to 10 minutes, the substrate heater 14 isenergized and the target plate 12 is raised thereby a temperature of 360to 380 C. This temperature 1s allowed to stabilize for about 30 minutesbefore the coating operation is commenced.

The lead oxide coating material is placed in a crucxble 20 andarrangement is made for heating it by induction through the energizationof the coil 21. The crucible temperature is raised slowly over a periodof l to 20 minutes. At 830 C., the lead oxide commences to vaporize.Desirably the temperature is raised to a maximum of about 980 C., whichis maintained for approximately minutes. The induction heater 21 is thendeenergized and the target plate 12 is allowed to cool gradually in theoxygen atmosphere'for about 30 minutes. Thereupon the chamber 13 isopened by removing its cover 22 and whenthe target plate 12 is removed,it will have a velvety appearing red oxide coating of a red orange colorand composed of a random orientation of red and yellow crystals. Thetarget is placed immediately in a 600 C. oven and baked in air for about90 minutes.

The lead oxide is thereby converted to the yellow crystalline form. f

Desirably the target is then replaced in the vacuum chamber 13, thecover 22 is sealed over it, and the coating operation is repeated toprovide a second layer of lead oxide as shown at in Fig. 2. Theprocedure is identical with that above described, including baking inair for about 90 minutes after removal of the target plate from thevacuum chamber.

The combined thickness of the two layers 9 and 10 of lead oxide is ofthe approximate order of 0.2 mm. and the color, in consequence of theair bake, is now yellow. The yellow crystalline lead oxide has aresistance of 1l)11 ohm-cm. and gives a faster response to X-radiationthan the red form.

During the condensation of the lead oxide on the substrate 12 in chamber13, it is important to maintain a substrate temperature of 200 C. orgreater, 350 C. or greater being preferred to get optimum results. Atlower substrate or target plate temperatures, the resistivity of thelead oxide coating is lower a'nd the image decay time is higher. Thelatter is particularly important, as there is a pronounced reduction indecay time as the substrate temperature during lead oxide deposit isincreased to and above 200 C. A reduced decay time reduces blurring andfacilitates X-ray observation of moving objects.

The baking of the lead oxide and its conversion to the yellowcrystalline form necessarily results in the oxidation of the substratematerial at the interface. Consequently itis important that the selectedsubstrate metals be metals whose oxides are so formed as to provide thedesired uniformity of distribution and have the desired relatively highelectrical conductivity, whereby to achieve the objectives of thepresent invention.

The oxides of the components of the stainless steel alloy, and themetals suggested as alternatives for use in the substrate, all haveresistance below the desired maximum of l0s ohm-cm. Moreover, the oxidesshow a uniformity of resistance throughout the interface markedlysuperior to that found in the oxides of the aluminum surfaces heretoforeemployed to support the lead oxide. Thus, blemishes and spots such asresult from variations in relative conductivity in previously knowntargets are wholly or substantially eliminated.

The faster and higher response of the yellow lead oxide to X-radiation,in combination with the constant relative conductivity at the interfaceproduces sharper images, and particularly unblurred images of movingobjects,

I mase are less persistent and blurring of images of movmg objects iseliminated or greatly reduced. Moreover, the images are intensied byphoto emission from the substrate as pointed out above.

We claim:

1. A target for converting X-radiation into an electrical signalbypmeans of photo-conductivity and comprising a substrate plate ofelectrically conductive metal having a face portion and aphotoconductive layer of lead oxide adherent to said face portion, theelectrical conductivity of the said face portion being greatly in excessof the conductivity of the photoconductive layer.

2. A target as set forth in claim 1 in which the face portion isuniformly oxidized and comprises a metal selected from a group whichincludes stainless steel, chromium, nickel and copper, and the leadoxide comprises a yellow crystalline oxide of lead.

3. In a target for converting X-radiation into photoconductive action, asupport plate comprising relatively light metal ofself-supporting'thickness and good electrical conductivity havingstainless steel bonded to one of its faces in a layer too thin to beself-supporting in use, and a layer of lead oxide on an exposed face ofthe stainless steel and forming a photoconductor, the coeicient ofthermal expansion of the lead oxide and the stainless steel beingsonearly identical as to avoid cracking o the lead oxide in the course ofchanges of temperature aecting the target.

4. In a target for converting X-radiation into photoconductive action, aplate providing mechanical support and comprising relatively light metalof good electrical conductivity, said plate having a facial coating of aheavier metal uniformly superlicially oxidized, and a layer ofphotoconductive yellow lead oxide on the superficial oxide ofheaviermetal, the oxide of the heavy coating metal becoming very substantiallymore conductive electrically than the yellow lead oxide and havingapproximately the same co-efiicient of thermal expansion, relativeconductivity of the oxidized surface of the heavier metal coating andthe oxide being substantially uniform throughout the interfacetherebetween.

5. The target .of claim 4 in which the coating of heavier metal is lessconductive to X-radiation than the support plate and filters soft andscattered rays of X-radiation, thereby intensifying the photoconductiveresponse of the yellow lead oxide layer when the target is subjected toX-radiation.

6. The target defined in claim 5 in which the coating metal is selectedfrom the group consisting of stainless steel, copper, nickel, chromiumand alloys of steel, copper, nickel or chromium.

7. The target of claim 5 in which the coating metal comprises stainlesssteel of the type defined as AISI type 430.

8. The method of making a target plate for converting X-radiation intophoto-conductive action, which method comprises plating a substrate oflightweight and selfsustaining thickness and capable of good X-raytransmission with a ply of materially heavier metal to a. thicknessinadequate for self-support, whereby the plated ply is dependent on thesubstrate for support, and subsequently applying lead oxide to thesurface of said ply, and baking the substrate and said ply and coating,producing an oxide on the said ply which has materially greaterelectrical conductivity than the lead oxide, and concurrently convertingthe lead oxide to yellow lead oxide.

9. The method recited in claim 8 in which said ply comprises stainlesssteel.

References Cited in the tile of this patent UNITED STATES PATENTS Lewiset a1. Aug. l5, 1939 Sheldon May 22, 1956 UNITED STATES PATENT OFFICECERTIFICATE 0F CORRECTION Patent No. 2,886,726 Mayvlz, 1959 HaroldBerger et al.

It is hereby certified that error appears in the printed specification lof the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below.

l Column 4, line 36, for "becoming" read being signed and sealed thislthday of september 1959.

(SEAL) Attest;

KARL H. ACLINE ROBERT C. WATSON Attesting Oicer Commissioner of Patents

1. A TARGET FOR CONVERTING X-RADIATION INTO AN ELECTRICAL SIGNAL BYMEANS OF PHOTO-CONDUCTIVITY AND COMPRISING A SUBSTRATE PLATE OFELECTRICALLY CONDUCTIVE METAL HAVING A FACE PORTION AND APHOTOCONDUCTIVE LAYER OF LEAD OXIDE ADHERENT TO SAID FACE PORTION, THEELECTRICAL CONDUCTIVITY OF THE SAID FACE PORTION BEING GREATLY IN EXCESSOF THE CONDUCTIVITY OF THE PHOTOCONDUCTIVE LAYER.